It was an intriguing news release: “BREAKING: FBI Opens Investigation into No on 37 Shenanigans.”

But there was one problem with it: It wasn’t true.

The Yes on 37 campaign, which wants to require new labels on foods with genetically engineered ingredients sold in California, has raised concerns about various mailers that the “no” side has sent out to voters.

Chief among them is the use of an official seal of the U.S. Food and Drug Administration on a No on 37 mailer. It made it seem as if the FDA, which doesn’t believe genetically modified food needs to be labeled, has endorsed the No on 37 campaign. It hasn’t.

The “yes” campaign recently sent a letter to the U.S. Department of Justice requesting a criminal investigation of the “no” campaign for possible fraudulent misuse of the FDA seal.

On Thursday, an FBI agent from the Sacramento field office apparently called the Yes on 37 campaign and said the agency was looking into the matter. That prompted the campaign to organize a conference call with reporters Friday morning to talk about the fact that the “U.S. Department of Justice is actively pursuing a criminal investigation of our complaint.”

But on Friday, U.S. Attorney Benjamin Wagner of the Eastern District of California issued a rare statement directly contradicting the claim.

“It has come to the attention of this office that some persons have suggested that this office has an active investigation relating to the use of the FDA seal in an advertisement relating to a campaign involving Proposition 37,” the statement said. “Approximately two weeks ago, the U.S. attorney’s office received a letter complaining about the misuse of the official seal of the FDA in a campaign advertisement. We have referred the matter to the FDA for any action they believe is appropriate. Neither the FBI nor this office has a pending investigation related to this matter.”

During the conference call, several reporters asked Yes on 37 campaign officials what led them to believe that an active investigation was under way. The officials said that an FBI agent named Jason Jones called the campaign Thursday and said he was looking into the facts behind the allegation.

“It seems to us that there is an investigation under way,” said Ken Cook, president of the nonprofit Environmental Working Group and one of several people on the call. “We are waiting for the FDA to issue a statement.”

The Yes on 37 campaign has enormous grass-roots support from natural-food companies and consumers who favor organic food and support new labels for genetically engineered ingredients. But opponents, led by large chemical and processed-food companies, have spent more than $45 million to defeat the measure, compared with about $8 million raised by the “yes” side.

At least $26 million of the money raised by the “no” side has gone to radio, television and Web advertising, causing support for the measure to drop in the polls.

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